D-mannitol is a six-carbon sugar alcohol, which is about half as sweet as sucrose. It is found in small quantities in most fruits and vegetables (Ikawa et al., 1972; Bär, 1985). Mannitol is widely used in various industrial applications. The largest application of mannitol is as a food additive (E421), where it is used e.g. as a sweet tasting bodying and texturing agent (Soetaert at al., 1999). Crystalline mannitol is non-sticky, i.e. it prevents moisture absorption, and is therefore useful as coating material of e.g. chewing gums and pharmaceuticals. In medicine, mannitol is used as osmotic diuretic for intoxication therapy and in surgery, parenteral mannitol solutions are applied to prevent kidney failure (Soetaert at al., 1999). Mannitol is also used in brain surgery to reduce cerebral edema.
At present, commercial production of mannitol is done by catalytic hydrogenation of invert sugar with the co-production of another sugar alcohol, sorbitol. Typically, the hydrogenation of a 50/50-fructose/glucose mixture results in a 30/70 mixture of mannitol and sorbitol (Soetaert at al., 1999). Besides the fact that mannitol is the by-product of the chemical production process and thus liable to supply problems, it is also relatively difficult to separate from sorbitol. In contrast to most sugars and other sugar alcohols mannitol dissolves poorly in water (13% (w/w) at 14° C. (Perry et al., 1997)). Cooling crystallization is therefore commonly used as a separation method for mannitol. However, according to Takemura et al. (1978) the yield of crystalline mannitol in the chemical process is still only approximately 17% (w/w) based on the initial sugar substrates.
In order to improve the total yield of mannitol it would be advantageous to develop a process with mannitol as the main product and with no sorbitol formation. Some alternative processes based on the use of microbes have been suggested in the literature. Yeast, fungi, and LAB especially, are able to effectively produce mannitol without co-formation of sorbitol (Itoh et al. 1992). Among LAB only heterofermentative species are known to convert fructose into mannitol (Pilone et al. 1991; Axelsson, 1993; Soetaert et al. 1999). Species belonging to the genera Leuconostoc, Oenococcus and Lactobacillus particularly, have been reported to produce mannitol effectively. In addition to mannitol these microbes co-produce lactic and acetic acid, carbon dioxide and ethanol. These by-products are, however, easily separable from mannitol.
Soetaert and co-workers have studied the bioconversion of fructose into mannitol with free cells of Leuconostoc pseudomesenteroides ATCC-12291 (Soetaert et al., 1994). Using a fed-batch cultivation protocol they reached a maximum volumetric productivity of 11 g mannitol/L/h and a conversion efficiency of approximately 94 mole-%. Recently, Korakli et al. (2000) reported a 100% conversion efficiency with Lactobacillus sanfranciscensis LTH-2590. Other heterofermentative LAB reported to be good producers of mannitol include Leuconostoc mesenteroides, Oenococcus oeni, Lactobacillus brevis, Lactobacillus buchneri and Lactobacillus fermentum (Pimentel et al., 1994; Salon et al. 1994; Erten, 1998; Soetaert et al. 1999).
In JP62239995, Hideyuki et al. (1987) used free cells of Lb. brevis. The volumetric mannitol productivity achieved in batch fermentation was 2.4 g/L/h.
EP0486024 and EP0683152 describe a strain named Lb. sp. B001 with volumetric mannitol productivities of 6.4 g/L/h in a free cell batch fermentation (Itoh et al., 1992; Itoh et al., 1995).
More recently, Ojamo et al. (2000) have submitted a patent application for a process for the production of mannitol by immobilized LAB. In this process the average volumetric mannitol productivity and conversion efficiency achieved were approximately 20 g/L/h and 85%, respectively. A low-nutrient medium was used which considerably lowers the production costs. Immobilization also enables the re-use of cell biomass for successive batch fermentations.
These inventions have not yet replaced the conventional hydrogenation process. The free cell bioconversion processes described to date are not entirely suitable for industrial scale production. Volumetric productivities in the range of 20 g/L/h, as achieved with the immobilization process, should however, be adequate for profitable production. In order to further develop the features of the bioconversion alternative, factors such as equipment investment costs, robustness of the process, medium composition (raw material costs), and mannitol yields must be considered and improved. The goal of the present invention is to overcome the prior disadvantages, such as the low productivities obtained with the free cell bioconversion systems and the low mannitol yields characteristic for all available bioconversion systems. Thus, the goal of the present invention is to develop a bioconversion process, which is feasible both technically and economically.